The invention relates generally to semiconductor integrated circuit (IC) devices and more particularly, it relates to driver circuits with load discharge detection particularly suitable for use with electro-luminescence (EL) lamps.
EL-lamps are used for backlighting of LCD displays in mobile phones and PDAs. An EL-lamp has a uniform brightness along the lamp area. This is an advantage with respect to backlighting using LEDs because they create bright spots. Since the EL-lamp behaves like a large capacitor, it is driven by an AC voltage of 200-400 Hz. The amplitude of the AC voltage (100-250V) determines the brightness.
EL lamp drivers play an important role in ensuring the uniform brightness of the EL lamp. Conventional EL lamp drivers attempt to achieve the lamp uniform brightness in various ways. Some EL lamp drivers, e.g., Supertex HV824 available from Supertex located in Sunnyvale, Calif., attempt to reduce smoothly the lamp voltage before the opposite polarity of the lamp voltage is built up, without using a controlled constant discharge current. The opposite voltage is connected to the lamp directly which result in abrupt voltage changes. These abrupt voltage changes can result in sound coming from the lamp or electrical disturbance in other parts of the application.
Other EL lamp driver, e.g., the D371 EL lamp driver available from Durel Corporation in Chandler, Ariz. uses a controlled constant discharge current to discharge the EL lamp. However, the time needed for the discharge of the lamp however is not measured but has a fixed relation with the lamp frequency. In most applications, this discharge time will be longer than necessary. This can cause the performance of the EL lamp to degrade, resulting in lower brightness.
Therefore, there is a need for a driver circuit with superior performance.
The present invention provides a solution that addresses the limiting factors of the conventional driver circuits and has the following advantages. The invention automatically determines the optimal time needed for the discharge of the load. The discharge time adapts itself to different load sizes or load voltages. Also, a minimum amount of time is used to discharge the load, so that a maximum amount of time is available for charging the load at a predetermined frequency. As a result, a higher performance can be achieved. For example, when the invention is used with an EL lamp, a higher brightness can be achieved.
According to one embodiment of the invention, there is provided a driver circuit for driving a load. The driver circuit comprises a sensing circuit that is configured to detect whether the load is substantially discharged and provide an end-of-discharge signal upon detecting a substantial discharge of the load. The driver circuit also includes a controller, operably connected to the sensing circuit, that is configured charging and discharging of the load. The controller starts a new charging cycle to enable charging of the load in response to the end-of-discharge signal.